(1) Field of the Invention
This invention relates to polyisocyanate compositions comprising (a) polyisocyanate, and (b) a monomeric carbodiimide. The polyisocyanate compositions are particularly useful in phenolic urethane foundry binder systems. The invention also relates to foundry mixes prepared with (a) a phenolic urethane binder containing the polyisocyanate composition, and (b) a foundry aggregate, as well as foundry shapes prepared by the no-bake and cold-box processes, which are used to make metal castings.
(2) Description of the Related Art
One of the major processes used in the foundry industry for making metal parts is sand casting. In sand casting, disposable foundry shapes (usually characterized as molds and cores) are made by shaping and curing a foundry binder system that is a mixture of sand and an organic or inorganic binder. The binder is used to strengthen the molds and cores.
Two of the major processes used in sand casting for making molds and cores are the no-bake process and the cold-box process. In the no-bake process, a liquid curing agent is mixed with an aggregate and shaped to produce a cured mold and/or core. In the cold-box process, a gaseous curing agent is passed through a compacted shaped mix to produce a cured mold and/or core. Phenolic urethane binders, cured with a gaseous tertiary amine catalyst, are often used in the cold-box process to hold shaped foundry aggregate together as a mold or core. See for example U.S. Pat. No. 3,409,579. The phenolic urethane binder system usually consists of a phenolic resin component and polyisocyanate component which are mixed with sand prior to compacting and curing to form a foundry binder system.
Among other things, the binder must have a low viscosity, be gel-free, remain stable under storage and use conditions, and cure efficiently. The foundry binder system made by mixing sand with the binder must have adequate benchlife or the mix will not shape and cure properly. The cores and molds made with the binders must have adequate tensile strengths under normal and humid conditions, and release effectively from the pattern. Binders which meet all of these requirements are not easy to develop.
One of the problems associated with using phenolic urethane binders is that the polyisocyanate component is known to react with even minor amounts of water, which might come from the moisture in air during handling or use, raw materials, and particularly from exposure to atmospheric conditions when used during hot and humid summer months. Hydrolysis of the polyisocyanate results in formation of urea compounds, which degrade the performance of the binder and results in the formation of sludge during storage. This reaction with moisture can cause an increase of the viscosity of the polyisocyanate component and seriously impede the performance of the binder when it is used to bond a foundry aggregate.
All citations referred to under this description of the “Related Art” and in the “Detailed Description of the Invention” are expressly incorporated by reference.